Vibratory conveyors are used for the transport of free flowing materials, such as crisps and light snacks.
A typical known conveyor is as shown in FIG. 1 of the accompanying drawings and consists of a tray and base with flexible members between the two. The tray consists of a flat longitudinal section with vertical sidewalls. The base is a heavy mass which minimises the transmission of vibration to the support structure (not shown). The flexible members are leaf springs which allow the tray to vibrate when it is excited by a drive. Typically, the tray vibrates at a frequency of 25 Hz and with a stroke of 4 mm.
Because the leaf springs are set at an angle, the motion is essentially horizontal although there is a small vertical component. Hence, the product is projected both forwards and upwards with each oscillation. This produces a flow of product along the length of the conveyor, the arrow indicating the direction of product flow, as also in the other Figures. The speed of flow can be controlled by adjustment of the stroke length of the oscillations.
A conveying system consists of a number of such conveyors. These can be arranged to form a so-called "prioritised" system. The arrangement of a prioritised system is as FIG. 2, except that the illustrated photo eyes P1, P2 and P3, which are described below, are absent. In this system there is a feed onto a first conveyor (C1) and thereafter either to two other conveyors (C2 and C3) or via gates (G1, G2 and G3) which, when open, allow product to fall below to supply packaging stations 1, 2 and 3 (not shown). When the gates are closed product simply passes across them; for example when gate G1 is closed product passes across it from conveyor C1 to conveyor C2. In its simplest form there is a continuous feed of product at the infeed of conveyor C1 and that conveyor runs continuously. If product is needed by station 1 then gate G1 opens and all subsequent conveyors stop, regardless of whether any other station requires product. Only when station 1 is satisfied is product fed to another station in the line and then all conveyors upstream of that station run. So, for example, if packaging station 2 needs product, then gate G2 opens and conveyors C1 and C2 run. Hence the station at the start of the line has highest priority and the priority reduces along the line. In the event of no station requiring product, all the conveyors run and the product is removed from the end of the line.
With more sophisticated control it is possible to define the station priority order so it is not necessarily highest to lowest from the beginning to the end of the line, although still no more than one station is fed at once.
When the supply of product to the system is continuous but the demand is intermittent, it is desirable to have a means of product storage. So-called "semi-prioritised" control allows this storage to take place in the conveyors themselves. This results in gentle product handling and eliminates the need for additional equipment which would otherwise be required for storage purposes.
A typical semi-prioritised system is shown in plan view in FIG. 2 of the accompanying drawings. At the infeed end of each conveyor is a photo eye (P1, P2 and P3) monitoring the product as it is fed onto that conveyor. If more than one packaging station requires product then all the appropriate gates open and all conveyors upstream of those gates run. This has the disadvantageous side effect that gaps are generated in the product flow on the conveyors if more than one packaging station needs product at the same time. For example, if the packaging stations fed by gates G1 and G2 both require product, and both gates are opened, conveyors C1 and C2 both run and feed product respectively to the two gates. As no product is entering the infeed end of conveyor C2 this has the effect that the conveyor C2 progressively empties, beginning at the infeed end, so creating the gap just referred to. In consequence, even if gate G1 is subsequently closed, there will be a period of time for which gate G2 does not receive any product.
What has just been described is to some extent a theoretical idealisation. What in practice tends to happen is that as the product travels along the conveyor the product at either end of the gap spreads out to create a region where there is product of reduced depth, and a point may be reached where the gap disappears altogether. This region of reduced depth is also disadvantageous, at least where the depth falls below a certain level. Thus, for certain products it is desirable to retain heat from the cooking process within the product until it is packed, and this is impaired if the depth of product is substantially reduced. There may also be other disadvantages. These include increased product breakage, increased separation of large and small pieces of product, and, where flavouring is added to the product after cooking (as, for example, with flavoured crisps) less retention of the flavouring in the product.
In storage mode, when no packaging station requires product, product is still fed onto the first conveyor (C1), for example from a cooker. However, conveyor C1 remains stationary until a photo eye P1 detects product, and then it runs at a reduced amplitude, which is typically around 50% of the normal, higher amplitude. The photo eye is set at a level so that the depth of product almost reaches the top of the conveyor's sidewall, before the conveyor runs. The consequence of this is that conveyor C1 runs intermittently as product is introduced to it and progressively fills to a high level from its infeed end to its outfeed end as product is stored on it.
In storage mode, whenever conveyor C1 runs, product is fed onto conveyor C2 and this operates in the same way so that it runs at a reduced amplitude whenever photo eye P2 detects product. Likewise with conveyor C3 and photo eye P3. Hence the system progressively fills to a greater depth from its infeed end as product is stored. Ultimately, when all the conveyors are full and there is no further storage capacity, all the conveyors run at a reduced amplitude, and product is removed from the outfeed end of conveyor C3.
Whenever a packaging station calls for product, control reverts to that described previously, where all conveyors upstream of that packaging station run at the higher amplitude.
The storage capacity of the system is the difference between the product depth when running at the higher amplitude and the product depth when running at the reduced amplitude.
With electronic conveyor drives, it is possible to control the amplitude of vibration of each conveyor using an analogue signal, such as a 4-20 mA signal from a programmable logic controller.